Term long-run marginal cost curve Definition: A graphical representation of the relationship between long-run marginal cost and the quantity of output produced. Like other marginal curves, the long-run marginal cost curve follows the average-marginal rule relative to the long-run average cost curve. In all outward appearance, the long-run marginal cost curve looks very much like the short-run marginal cost, that is, it is U-shaped. However, the U-shape is attributable to returns to scale rather than increasing and decreasing marginal returns.